The present invention relates to a method and apparatus for the preparation of a silica glass preform as a precursor of optical fibers. More particularly, the invention relates to a method and apparatus by which a large-size porous silica glass preform can be sintered and vitrified into a transparent silica glass preform with good efficiency.
While a transparent silica glass preform as a precursor of silica glass optical fibers is prepared by sintering and vitrification of a porous silica glass preform at a temperature of 1300.degree. to 1500.degree. C., such a porous silica glass preform is produced mainly by the so-called VAD (vapor-phase axial deposition) method, in which a gaseous feed of a volatile silicon-containing compound such as silicon tetrachloride with optional admixture of a dopant such as germanium tetrachloride is subjected to flame hydrolysis in an oxyhydrogen flame to form fine particles of silica which are deposited on the carrier or a vertically held seed rod made from fused silica glass and connected to the lower end of a suspender rod in the form of a porous silica body which grows in the axial or vertical direction as the deposition of the silica particles proceeds to give an elongated porous rod of silica glass to be subsequently subjected to sintering and vitrification in a vertical tubular sintering furnace to give a transparent silica glass preform. This sintering process is carried out by gradually lowering the porous silica glass rod suspended at the lower end of the vertical suspender rod through the seed rod under rotation into the sintering furnace where the porous body is sintered and vitrified followed by pulling-up of the thus vitrified glass body above the sintering furnace. Namely, an apparatus is indispensable in the sintering process of a porous silica glass rod into a transparent silica glass preform for lowering an elongated porous silica glass rod as vertically suspended into the sintering furnace and pulling up the vitrified body out of the sintering furnace.
In order to comply with the rapidly growing demand for optical fibers of silica glass in recent years, the porous silica glass preform in the form of a rod prepared by the above described method is also required to have an increasingly large size having a length of, for example, 2000 mm or even longer as prepared in a reactor furnace for the flame hydrolysis of a volatile silicon compound. Assuming that the porous silica glass body in the form of a rod has a length of 2000 mm as formed in the reactor furnace and the sintered and vitrified silica glass preform to be pulled up from the sintering furnace has a length of 1000 mm, the suspender rod to suspend the porous body at the lower end thereof must have a length of 2500 mm so that the apparatus for lowering the porous body into the sintering furnace and pulling up the sintered body therefrom must have an effective stroke of at least 4800 mm and the overall height of the sintering apparatus sometimes exceeds 10 meters.
It may be too much to say that an apparatus having a so large height is accompanied by several disadvantages. Firstly, the cost for the construction of such a large apparatus is necessarily very high as compared with the cost for a more compact apparatus. Secondly, a serious problem is caused relative to the accuracy of the rotating movement of the suspender rod which is rotated necessarily with eccentricity more or less when the length thereof is great since the suspender rod is held by a carriage only at the upper part. Needless to say, eccentric rotation of the suspender rod seriously affects the uniformity of the sintered and vitrified silica glass preform. Thirdly, the plant house in which the apparatus is installed must be high enough as a matter of course which is constructed with a very large construction investment and requires a large amount of maintenance costs. In sum, the height of the apparatus for lowering a suspended porous silica glass body into the sintering furnace and pulling up the same after vitrification is one of the important determinant factors of the costs for the manufacture of optical fibers because of the large investment therefor and the limitation in the improvement of the productivity.